Uni-travelling-carrier (UTC) photodiodes are typically used in high speed optical telecommunication systems. Such devices typically operate based on converting an incoming light to electricity. A UTC typically comprises a p-doped absorption region in which incident photons are absorbed and generate free carriers (electrons and holes). This absorption layer is typically followed by a lightly doped transparent layer called collector. As photons absorption typically takes place in a p-doped layer, holes are majority carriers and only electrons are typically the active carriers. The photo-generated electrons diffuse through the p-doped region and reach the depleted collector region. By applying a reverse bias voltage to said collector region, electrons may be further accelerated and collected in an n-doped region. Thus creating a reverse current which is typically proportional to the incident optical power.
Since only electrons are used as active carriers, the UTCs typically can reach a higher speed and a higher saturation current as compared to conventional PIN photodiodes where both holes and electrons transit in a depleted region.